GolamMostafa:
However, I am at a loss to discover how the writer could miss a comma
IMHO both commas should have been omitted - and for the same reason. Taking the first sentence as an example ...
In a beginner/teaching environment, portability is desirable.
Presented that way it implies some disconnect between the first and second parts when, in fact, the author did not intend any. Perhaps a better way of writing would have been
portability is desirable in a beginner/teaching environment
and then you can see that there is no place for a comma.
The same logic can be applied to the other sentence that has a comma.
I would not see an objection to a comma after the word sequitur although I think the meaning is perfectly clear without one.
I have a bigger problem with the middle sentence - I suspect the author did not understand the meaning and usage of "non sequitur". IMHO the last two sentences should have been written
But in a production/commercial environment the overhead associated with portability may come at a too great cost as the code is always going to be optimized for the destination platform
...R